Development of acid-free chitosan films in food coating applications: provolone cheese as a case study

Introduction Chitosan has been extensively explored in food coatings. Still, its practical application is largely hampered by its conventional wet processing in acetic acid, whose residuals negatively impact food quality and safety. We propose a new method to formulate chitosan coatings for food applications by avoiding organic acid processing and validate them on a cheese model. Material and Methods The procedure entails modifying a previously reported process based on HCl chitosan treatment and neutralising the resulting gel. The obtained chitosan is solubilised in water using carbonic acid that forms in situ by dissolving carbon dioxide gas. The reversibility of water carbonation allows for easy removal of carbonic acid residues, resulting in acid-free chitosan films and coatings. We characterised the chitosan solutions (rheology) and the derived films' properties (water stability, barrier and optical properties). Then, we assessed the performance of the coating on Provolone cheese as a food model (mass transfer and texture profiles over 14 days). In addition, the migration of acetic acid into the food matrix that occurs in conventional chitosan coatings was measured. Results Viscosity measurement showed a strong shear thinning behaviour of chitosan solutions in carbonic acid, facilitating the coating processes. The acid-free chitosan films have superior water stability and barrier properties compared to commonly acetic acid ones. When applied to Provolone cheese, acid-free chitosan coatings showed the ability to preserve cheese properties comparable to commercial PVAc and chitosan film from conventional aqueous acetic acid. Still, it was demonstrated that the residual acid in acetic acid-processed chitosan exceeded the specific migration limit for packaging in contact with food and the odour detection limit. Discussion In conclusion, processing chitosan via wet approaches that eliminate acetic acid has been proven to be a promising alternative to conventional dissolution. This method further enhances the properties of chitosan films. It enables them to meet stringent food quality requirements and safety regulations, paving the way for an effective application of chitosan in future food contact applications.